1. Field of the Invention
This invention relates to transformers and more particularly to a compensated transformer for use in interfacing a transmission device to a cable facility in a manner such that a predetermined one of a number of selectable impedances is provided to the cable facility.
2. Description of the Prior Art
Transmission devices often interface a cable facility through a transformer. The use of a transformer is desirable in that it provides isolation between the cable facility and the transmission device. The transmission devices are often required to have selectable input and output impedances so that the same device may be usable with both non-loaded and loaded cables. A common requirement for voice frequency transmission devices is to have such impedances selectable at 150 and 600 ohms in order for the device to interface with non-loaded cable and selectable at 1200 ohms in order for the device to interface with loaded cable.
In addition to the selectable input and output impedances described above it is also desirable that the envelope delay distortion to the signal passing through the transformer be held to less than 50 microseconds at 300 Hz. In order for the transformer to both interface an impedance of 1200 ohms and have the above desired envelope delay distortion performance it is necessary to provide an inductance of greater than three (3) henries in the transformer. The transformer may be implemented using either a pot core or a laminated core using nickel iron material. It is desirable to implement the transformer using a pot core as transformers of that type provide both good cross talk isolation and a predictable amount of inductance for a given number of turns. In addition, the pot core type of transformer is much less expensive than the nickel iron lamination type. Unfortunately, since the permeability of pot core material is less than that of nickel iron laminations, for a given transformer size of the pot core type a large number of turns must be wound in the transformer in order to provide the above inductance.
For small transformers the winding resistance resulting from those large number of turns, while tolerable at 1200 and 600 ohms, becomes quite intolerable at 150 ohms when a simple, untapped transformer is used. The winding resistance may be so large that it exceeds the required 150 ohm impedance making it impossible to have that impedance. Even if the winding resistance does not exceed the required impedance, it may so dominate that impedance that the winding resistance forms the bulk of the 150 ohm impedance. In this case the transformer is extremely lossy.
A typical transformer specification permits a 10 to 15 percent allowable variation on winding resistance independent of any changes in resistance with temperature. This gives rise to a wide range of variations in winding resistance for pot core transformers of the same size. It is therefore necessary to provide a factory gain compensation adjustment in the transmission device during its manufacture. This wide variation in winding resistance also affects return loss. These variations in return loss can not be compensated for without an additional factory adjustment. The prior art solution to out of tolerance return loss is usually to replace the transformer. In addition, the return loss and the impedance of the transformer are extremely temperature sensitive. The copper which is used to make the needed turns changes its resistivity with changes in temperature at the well known rate of 0.39%/.degree.C. Thus, the use of small pot core transformers to provide both the desired impedance matching as well as the desired limits on envelope delay distortion gives rise to problems.